Someone prodded me into drawing up a timeline of the Emacs family
tree. Let me know if you have any additions/corrections.

This isn't intended to be a complete list of everything that has
ever called itself Emacs -- as Craig Finseth's periodic
posting of emacs
implementations shows, that would be much larger than this.

However, I think this is fairly accurate for the
GNU/Lucid/X Emacs part of the world, and their important
predecessors.
(An explanation of how and why the FSF Emacs / Lucid Emacs
split came about is over here.)